Because of high energy density, the sealed type secondary batteries, lithium-ion secondary batteries among others, are being increasingly used as the power source for driving compact portable appliances such as video camera, portable telephone, personal computer, etc. It is essential for the lithium-ion secondary batteries to have a safety device containing an electric circuit in order to protect the battery against over-charging, over-discharging and excessive electric current. Therefore, such batteries are normally packed in a so-called battery package incorporating an electric circuit working as the safety circuit, designed for installation in an appliance.
FIG. 1 is a conceptual drawing of a battery package containing lithium-ion secondary batteries; where, numeral 1 denotes a case to house cylindrical lithium-ion secondary batteries 2 and 3 designed to have a shape suitable for installation in a certain specific appliance, 4 is a safety circuit disposed between the lithium-ion secondary batteries 2, 3 and a terminal of the case 1.
FIG. 17 is a block diagram of a conventional safety circuit 4; where, the positive voltage supplied from a battery charger flows through a plus terminal 5, a PTC element 6, lithium-ion secondary batteries 3, 2, FETs 7, 8, a thermal fuse 9 and terminal 10, ultimately to the negative end of battery charger. The lithium-ion secondary batteries 2, 3 are charged in the above manner.
The lithium-ion batteries 2, 3 discharge through PTC element 6, plus terminal 5, a plus and a minus terminals of an appliance, minus terminal 10, thermal fuse 9 and FETs 8, 7. Voltage detectors 11, 12 for watching battery voltage are coupled to a control circuit 13; which circuit turns FET 8 OFF when the battery voltage goes beyond a specified value to prevent over-charging, and turns FET 7 OFF when the battery voltage goes lower than a specified value to cut the electric current for preventing over-discharging. A voltage detector 14 which detects the source-drain voltage of the FET 7 is coupled to the control circuit 13; which circuit turns the FETs 7 and 8 OFF to cut the electric current when an excessive electric current goes between the plus terminal 5 and minus terminal 10 due to short-circuiting etc. in outside. A thermistor 15 is for monitoring battery temperature from a battery charger, or an appliance, through a terminal 16.
As described in the above, a conventional battery package contains in its case 1 a safety circuit 4 comprised of preventive means against over-charging, over-discharging of battery, electric current cutting means against excessive electric current flow, battery temperature monitor, etc. mounted on a printed circuit board.
The conventional battery package, however, is accompanied with following drawbacks.
1) Most of the battery packages are packed in their case 1 of specific design dedicated to each of respective shapes of appliances they serve, for the ease of installation. PA1 2) Dedicated battery charger is needed for each of the respective appliances, because case 1 has its own specific design dedicated to each of the respective appliances, and a battery charger has to comply with the shape of case 1. PA1 3) Safety circuit and structure of terminals adapted to the shape of an appliance occupy a substantially bulky space relative to the space for lithium-ion secondary batteries 2, 3. PA1 4) Lithium-ion secondary batteries or other secondary batteries alone are not readily available for ordinary consumers. The conventional battery packages lack versatility.